Beginning of songs are cut too much when using cue_cut

[smlz]

Hi, we are currently renewing our studio automation software and liquidsoap really safes our day. Thanks to everyone contributing to this fine piece of software.

I recently discovered a small problem when using cue_cut. We use silan (https://github.com/x42/silan) to detect of silent parts at the beginning and the end of audio files, and then use this information with the cue_cut operator. When the time of the cue_in point is small (< 0.04s, smaller than the frame length) which quite often is the case, not just a couple of milliseconds, but around half a second is cut off.

It looks like this has happened to others as well: https://forum.sourcefabric.org/discussion/16854/begin-of-songs-are-cut/p1

Tested with:

$ liquidsoap --version
Liquidsoap 1.2.0
Copyright (c) 2003-2016 Savonet team
Liquidsoap is open-source software, released under GNU General Public License.
See <http://liquidsoap.fm> for more information.

$ liquidsoap 'out(cue_cut(single("annotate:liq_cue_in=\"0.001\":test.flac")))'

I think that the fseek function in the file decoder forgets to account for the contents in the pre-buffer when seeking backwards. See https://github.com/savonet/liquidsoap/blob/1.2.1/src/decoder/decoder.ml#L429-L433

[smlz]

Thanks for the merge.

Just for the record: here's a quick work-around we use for the current versions of Liquidsoap:

def fix_cue_in(~cue_in_metadata='liq_cue_in', m) =
    cue_in = float_of_string(m[cue_in_metadata])
    if cue_in < 0.04 then
        [(cue_in_metadata, "0")]
    else
        []
    end
end

s = map_metadata(fix_cue_in, s)
# or
# s = map_metadata(fix_cue_in(cue_in_metadata="whatever_youre_calling_it"), s)

# and then
s = cue_cut(s)

I don't know, whether the frame size is always 1/25 of a second in Liquidsoap. So, the value of 0.04 might need to be adjusted according to your setup.